EFFECT OF POST-FRAGMENTATION VISCOUS SHEAR STRAIN ON PARTICLE SIZE DISTRIBUTION FOR DYNAMIC AND QUASI-STATIC FRAGMENTATION

Particle size distribution (PSD) analysis of fragmented minerals has been used to estimate energy sources or loading conditions. For example, three-dimensional D-value (negative slope in log-log plot of the cumulative PSD) ≥2.5 is considered high strain-rate dynamic loading whereas D-value <2.5 indicates low-energy, quasi-static loading. However, PSDs can be modified by subsequent deformation. The individual fragments of pulverized minerals observed in active fault damage zones near Earth’s surface show little or no relative rotation and shear displacement, so that the original crystal shapes are preserved. In contrast, coseismically fragmented/pulverized grains in seismogenic shear zones at the frictional-to-viscous transition (FVT) are commonly distorted due to post- and interseismic flow, the individual fragments being variably rotated and displaced relative to one another. Thus, caution should be taken when analyzing D-values of fragmented minerals with various aspect ratios. To evaluate whether PSD analysis of fragmented minerals at the FVT can still be used as an indicator of dynamic loading when the microstructure is modified by post- and interseismic viscous deformation, we analyze dynamically fragmented garnets with various aspect ratios from a seismogenic shear zone and compare the results with garnets fragmented during approximately steady-state creep in an aseismic shear zone. Results show three-dimensional D-values between 2.7 and 3.4 for the dynamically deformed rocks and less than 2.5 for the aseismically deformed rocks regardless of the aspect ratios of the deformed garnets. Microfracture patterns in garnet are also analyzed for both shear zones, with dynamic deformation leading to relatively higher intensity and weaker preferred orientation of microfractures. Our findings indicate that PSD may be a useful tool for evaluating energy sources or loading conditions of brittly fractured minerals irrespective of the degree of post-fragmentation shearing.